The crosslinking reaction, the extent of which affects the structural integrity of the epithelium, is catalyzed by transglutaminases (TGases) through the formation of N-epsilon(gamma-glutamyl) lysine isopeptide bonds. An understanding of the molecular features of these enzyme, their cellular regulation and identification of their protein substrates in the body epidermis and in mucosal epithelial cells will provide insight into the physiological role of the crosslinking process. Expression of TGase K and E and their variant mutant clones in E. coli has provided a means of determining structural requirement for generation of maximum catalytic activity. In each enzyme, there is a highly conserved sequence of amino acids essential for enzyme activity. In TGase K, an amino-terminal peptide, residues 1-57, regulates activation of the enzyme. Cleavage at residue 570 provides maximum catalytic activity. In TGase E, maximum activity is obtained by cleavage at residue 471. In both enzymes, a C-terminal peptide influences substrate specificity. Limited proteolysis of both enzymes by Ca++- dependent protease appears to be the source of regulation in differentiating keratinocytes. Blood clots from a tuberculosis patient on isoniazid treatment with hemorrhagic symptoms were rapidly lysed by plasmin. The plasma of this patient contained normal factor XIII levels but no crosslinking of fibrin occurred even though the isolated fibrinogen formed normal crosslinked fibrin clots. An IgG fraction from this plasma was found to be responsible for the inhibition of crosslinking. The basis of this unique and potentially important anti-body reaction is under further study.